1. Field of the Invention
This invention relates to a method of minimizing the irritating odor of an encapsulized solution that is produced by the residual formaldehyde. More particularly, this invention relates to a method of minimizing the malodor emitted when formaldehyde is used as one ingredient of a microcapsule wall.
2. Description of the Prior Art
In recent years, many encapsulation processes have been proposed in which microcapsule walls are made from a resin produced by addition condensation of formaldehyde and an amino compound or a phenol. Specific examples of these processes are disclosed in Japanese Patent Application (OPI) Nos. 84882/78 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), 84883/78, 84881/78, 66878/77, 144383/76, 9079/76, 42380/72, Japanese Patent Publication No. 18671/77 and U.S. Pat. No. 3,993,831.
However, problems are inherent in these prior art methods due to the presence of a large amount of residual formalin in the microcapsule dispersion. For example, production of a pressure-sensitive copying paper involves coating the paper with free formaldehyde as well as the capsules, and therefore, in each of the coating, processing and use stages, the formaldehyde evaporates to emit a characteristic irritating odor which does great harm to the health of workers on the production line and, therefore, is not acceptable from a physiological or hygienic management viewpoint.
One known method of minimizing the hazard of the residual formalin is to use a formaldehyde absorber which selectively absorbs the free formaldehyde. U.S. Pat. No. 3,447,945 teaches the use of urea or sodium sulfite, and Japanese Patent Application (OPI) No. 75676/76 hydroxylamine hydrochloride, as the formaldehyde absorber. These absorbers are effective to some extent in a method wherein a protective colloidal such as gelatin which hardens by reaction with formaldehyde and abounds in protective colloids is used in combination with a protective colloidal precipitant such as gum arabic to form a complex coacervate with which a hydrophobic material is encapsulized, and thereafter the protective colloidal such as gelatin is hardened by formaldehyde. However, the absorbers cannot be effectively applied to other encapsulization methods. For example, when, according to the method of Example 1 or 3 of U.S. Pat. No. 3,447,945, urea is added to the solution described in an Example of U.S. Pat. No. 4,001,140 which contained capsules having walls of urea-formaldehyde resin for the purpose of preventing the evaporation of the residual formaldehyde, the rate of reaction between urea and formaldehyde was slow and the tightness of the walls of the microcapsules decreased. When sodium sulfite was added in accordance with the method of Example 5, no adequate results were obtained. An attempt to improve the efficiency by adding more sodium sulfite resulted in rupture of the capsule walls causing the hydrophobic material to be released from the capsules and to be agglomerated to form a precipitate.